In recent years, from the point of view of energy saving and reduction in carbon dioxide emissions, white light sources using LED elements (light-emitting diodes) have been drawing attention. Compared to incandescent lamps using tungsten filaments, white light sources using LED elements can achieve longer operating life and energy saving. A conventional white light source using an LED element excites a YAG phosphor by using a blue LED element whose emission peak is in a range of 400 to 530 nm, and mixes a blue light emitted from the LED element and a yellow light emitted from the YAG phosphor, to thereby achieve white light.
White light sources using LED elements are used for a signal, a backlight of a liquid crystal display device, and a general lighting equipment such as an interior light. Regarding the conventional white light source using the blue LED element, a peak height of blue light emitted from the blue LED element in an emission spectrum of the white light source is high to be 1.5 times or more a peak height of yellow light emitted from the phosphor, and thus an effect of blue light is strong.
Meanwhile, as white light sources using LED elements become widely used, an adverse effect of the white light sources on human body is started to be concerned. As described above, the conventional white light source using the LED element has the strong emission peak of the blue LED element. The white light with the strong blue peak as above is light which is largely different from natural light. The natural light is sunlight.
By taking the effect of the white light source on the human body as described above into consideration, LED elements and phosphors with different emission peaks are combined to mix four types of emission peaks, to thereby provide white light with small deviation from a spectral luminous efficiency. The spectral luminous efficiency is defined by the CIE (Commission Internationale de l'Eclairage) as a standard spectral relative luminous efficacy V(λ), in which a sensitivity of eyes of human being with respect to light is referred to as a spectral luminous efficacy. Accordingly, the spectral luminous efficiency and the standard spectral relative luminous efficacy V(λ) have the same meaning. By taking the effect of the blue light on the human body into consideration, light within a range of 420 to 490 nm is controlled. It can be considered that a method as above provides an effect of suppressing a secretion of melatonin.
Meanwhile, a human being has a circadian rhythm. Basically, a human being lives under natural light, but, in a contemporary society, there are various life-styles such as long hours of indoor work and lives in which day and night are reversed. If a life with no exposure to natural light is continued for a long period of time, there is a concern that the circadian rhythm is disturbed, which exerts an adverse effect on human body.